If electric cars and portable electronic products adopt paper-thin power battery, the demand for paper-thin and high-efficient separator will increase dramatically. According to the forecast of market size and demand for global vehicle power battery, the demand will increase to 0.562 billion square meters (m2) and the price will increase to 2.2 USD/m2 by the end of 2013. Along with the rapid growth in the market of electric cars, the development of separator will depend on the market demand of vehicles and production scale.
Most separators for power battery currently available in the market are mainly porous film types. A thin-type single-layer or multi-layer film is formed by way of melt stretching, coating and drying, or hot pressing. Let the separator applied in lithium battery and provided by the Celgard, LLC of USA and the Ube Industries Ltd. of Japan be taken for example. Porous separator having a thickness less than 20 um is formed by melt stretching process. The flux of high polymer when extruded is crystallized under a tensile stress and forms a lamellae structure which is perpendicular to extrusion direction and is arranged in parallel. Then, the lamellar structure is processed with heat treatment to obtain a hard elastic material. Then, slit pores are formed when lamellae is separated after the polymer film is stretched. Then, the polymer film is processed with thermal setting treatment to form a microporous film. However, the above method for manufacturing a microporous film has following disadvantages. The pore size and the porosity are hard to control, and the horizontal strength of the microporous film is poor because the polymer film is stretched only in a vertical manner. Japanese companies, such as Asahi Kasei, Tonen, and Mitsui Chemicals, form a porous separator having a thickness less than 20 um by using thermally induced phase separation method. Details of thermally induced phase separation method are as follows. The polymer is dissolved in a solvent having high boiling point and low volatility at higher temperature to form a homogeneous solution. Next, as the temperature cools down, the solution generates liquid-solid phase separation or liquid-liquid phase separation. Then, the high boiling point solvent is extracted by a volatile reagent, and is further dried to obtain a high polymer micro-porous film having a specific shape and structure. Although it is easier to control the pore size, the pore size distribution and the porosity in the thermally induced phase separation method than in the method used by the Celgard, the thermally induced phase separation method requires a large volume of solvent during the manufacturing process and employs relatively complicated procedures.
The Teijin Ltd. and the Japan Vilene Co. Ltd. of Japan use a composite material and a pore forming material to form a porous separator (PET non-woven fabric) having a thickness less than 30 μm and an average pore size greater than 5 μm. The pore size is controlled by the pore forming material. The Mitsubishi Corporation of Japan uses fibers of different diameters to form a separator (PET non-woven fabric) having a thickness less than 30 μm and an average pore size>10 μm. The pore size is controlled by composite conditions. The Degussa AG of Germany forms a supporting material (PET non-woven fabric) by using single fiber web spinning technology in which the supporting material is coated with inorganic powder to form a separator having a thickness less than 20 μm. However, the single fiber web spinning technology has following disadvantages. The pore size and the porosity are hard to control, and the horizontal strength of the porous separator is poor because the porous separator is stretched only in a vertical manner. In addition, the pore size is controlled by the amount of inorganic powder and the binder to be greater than 5 μm. Although each of the above methods for manufacturing porous separator by way of stretching or adding an inorganic powder has its own advantages and disadvantages, these methods have the same problem of the pore size being too large (greater than 5 μm), and the porous separator, when the porous separator is applied in separator for power battery, cannot effectively separate the positive polarity and the negative polarity of the battery and will deteriorate the performance of the power battery.
Therefore, how to provide a separator for power battery which matches the needs of development and low cost and at the same time produce excellent separation effect has become a prominent task for the industries.